L5.3.1: Events at the Neuromuscular Joint

• Neuromuscular Junction

  ○ Where muscle contraction occurs.

  ○ Site where a motor neuron excites a muscle fiber.
• Motor Unit

  ○ Consists of one neuron and all the skeletal muscle fibers it stimulates
• Axons

  ○ Threadlike extension of a neuron that reaches the muscle, branching into a number of axon terminals.
• Nerve Impulses — Action Potentials

  ○ Travel from the brain or the spinal cord to trigger the contraction of skeletal muscles.
• Axon Terminal

  ○ Forms the junctions with the sarcolemma of a muscle fiber.

  ○ Contains synaptic vesicles filled with a chemical referred to as neurotransmitter.
• Acetylcholine — ACh

  ○ The specific neurotransmitter that stimulates skeletal muscle fibers.

  ○ Released by the motor neuron to initiate muscle contraction.
• Synaptic Cleft

  ○ The gap or gap between the nerve endings (axon terminal) and muscle fiber membranes.

  ○ Filled with interstitial fluid.

  ○ Site of release of the acetylcholine from the axon terminal.
• ACh Receptor

  ○ On the surface of the sarcolemma (plasma membrane.)

  ○ Acetylcholine binds to these receptor sites.
• [1] Nerve Impulse Travels from the Motor Neuron and Reaches the Axon Terminal

  ○ As it reaches the axon terminal, it creates changes in the permeability within the membrane of the axon terminal, resulting in the influx of calcium ions.
• [2] Calcium Channels Open and Calcium Enters the Terminal

  ○ Calcium entry causes some of the synaptic vesicles to fuse with the cell membrane, releasing the acetylcholine across the synaptic cleft through the process of exocytosis.
• [3] ACh Attachment to Membrane Receptors

  ○ Acetylcholine diffuses across the synaptic cleft and attaches to membrane receptors in highly folded regions of the sarcolemma.

  ○ Acetylcholine binds to the receptors present on the highly folded sarcolemma.
• [4] Sarcolemma Becomes More Permeable to Sodium and Calcium Ions.

  ○ Acetylcholine Binding = Change in Permeability.

  ○ The sarcolemma becomes temporarily permeable to sodium ions which rush into the muscle fiber, and to potassium ions which diffuses out of the muscle fiber.
• [4] Sarcolemma Becomes More Permeable to Sodium and Calcium Ions.

  ○ More Na+ enters than K+ leaves. This imbalance gives the cell interior an excess of positive ions, which reverses the resting electrical conditions of the sarcolemma (depolarization). It opens more channels that only allow Na+ entry.
• [4] Sarcolemma Becomes More Permeable to Sodium and Calcium Ions.

  ○ The electrical current (action potential) generated by the movement of ions becomes unstoppable. It travels over the entire surface of the sarcolemma, conducting impulse from one end of the cell to the other. The result is the contraction of muscle fiber.
• [6] The Enzyme Acetylcholinesterase Breaks Down ACh

  ○ The remaining acetylcholine is broken down by the acetylcholinesterase in the synaptic cleft, ending the stimulation of the muscle fiber.

  ○ Broken down into acetic acid and choline.
• [6] The Enzyme Acetylcholinesterase Breaks Down ACh

  ○ While the action potential is occurring, the enzyme acetylcholinesterase (AChE), present on the sarcolemma and in the synaptic cleft, breaks down acetylcholine to acetic acid and choline to prevent continued contraction of the muscle fiber in the absence of additional nerve impulses.

  ○ The muscle fiber relaxes until stimulated by the next round of acetylcholine release, so a single nerve impulse produces only one contraction.
• Comparing the action potential to a flame consuming a dry twig. The charring of the twig by the flame can be compared to the change in membrane permeability that allows sodium ions into the cell. When that part of the twig becomes hot enough (when enough sodium ions have entered the cell), the twig will suddenly burst into flame, and the flame will move along the twig (the action potential will be conducted along the entire length of the sarcolemma).